CONTRIBUTION OF OGT-ENCODED ALKYLTRANSFERASE TO RESISTANCE TO CHLOROETHYLNITROSOUREAS IN NUCLEOTIDE EXCISION REPAIR-DEFICIENT ESCHERICHIA-COLI

We investigated the relative contribution of the two Escherichia coli DNA alkyltransferases (ATases) to the increased sensitivity of ATase-d eficient bacteria to the mutagenic and lethal effects of chloroethylni trosoureas (CNU). The ogt-encoded protein was the principal determinan t in resistance to the mutagenic effects of CNU in E. coli, Thus, only when the ogt gene was inactivated was sensitivity to mutagenesis grea tly increased; the contribution of inactivation of the ada gene was re latively minor, Furthermore, induction of the adaptive response provid ed essentially no protection against CNU mutagenesis in either an ogt( +) or ogt(-) background, Finally, overexpression of the ogt gene into ogt(-)ada(-) double mutants provided the greatest protection against C NU; introduction of the full-length or truncated ada gene was protecti ve, but to a much lesser extent, Mammalian ATases were not as protecti ve against mutation induction by CNU as Ogt, even though they were app arently expressed at higher level, In order of effectiveness the ATase s ranked Ogt > human > truncated Ada = Ada > rat. This order was not o bserved in the protection against killing by 1-(2-chloroethyl)-3-cyclo hexyl-1-nitrosourea, where truncated Ada = human > Ogt > rat = Ada, Hi gher mutation frequency and toxicity were observed in uvr(-) mutants, suggesting that one or more of the potentially mutagenic and/or toxic lesions are also substrates for the excision repair proteins.